This happened before, although slightly less blatantly. When trains were terminating at Southampton due to the strike, the planned rail replacements from Southampton to Bournemouth were cancelled "because of rail strikes." The whole practice of pre-planned RRBs not running has got to stop. It makes absolutely no sense, is incredibly anti-passenger and really shows the industry has lost track of its entire purpose. TOCs should not be artificially creating disruption under any circumstances!
Sure people will ask why they aren't covering other lines but that doesn't justify cancelling them, and is something that probably should happen on a small scale.